A single-cylinder engine has no exhaust manifold and the exhaust pipe is directly connected to a cylinder head. A multi-cylinder engine uses an exhaust manifold to collect the exhaust gas discharged from each cylinder and convey it to an exhaust pipe.
The exhaust manifold has a structure similar to an intake manifold. There are many types, one of which brings the exhaust gas to the center of the exhaust manifold, while another sends it to one end of the exhaust manifold. Some engines having six or more cylinders divide the number of cylinders by two and use two exhaust manifolds, but regardless, the manifolds are designed to minimize the flow resistance and interference of the exhaust gas.
In addition to collecting the exhaust gas and sending it to an exhaust pipe, the exhaust manifold also sends the exhaust gas to a catalytic converter. In the converter, HC, CO, and NOx components, which are harmful to the environment, are converted to harmless H2O, CO2, and N2, respectively, through oxidation and reduction reactions.
The conventional exhaust manifolds, however, has problems because the exhaust gas collected from each runner is generally not uniformly distributed across an entire cross section of the catalytic converter. Usually, the exhaust is heavily directed to a limited area of the catalytic converter and the purification efficiency is decreased. Also, this causes the limited area of the catalyst to operate at a higher temperature than other areas, so that the lifetime of the catalytic converter is reduced.
A reason for these problems is that space limitations make it difficult for designers to adjust the path and angle of each runner in such a way that the exhaust gas from each cylinder is uniformly dispersed across the catalyst. One space limitation is the necessity to provide room for fitting the exhaust manifold to the cylinder head during assembly. This requires that the runners are installed leaving a working space for tools. Another limitation is the need to reduce interference between the exhaust manifold and other components in the engine compartment.